The application of conducting polymer actuators to the propulsion of vehicles is taught in U.S. patent application Ser. No. 09/130,500, filed Aug. 7, 1998, which is attached hereto and incorporated herein by reference. Conducting polymer actuators are attractive because they have the potential to overcome the low torque to mass ratio and the efficiency limitations of current propulsion technologies.
In many applications, actuators impart kinetic energy to devices. In automobiles, for example, combustion engines are employed to develop tremendous kinetic energy (typically, kinetic energy on the order of 400 kJ). This energy is not recovered and, particularly for "stop and go" city driving, reduces vehicle fuel efficiency. The ability to reuse developed kinetic energy is an important consideration in choosing the next generation of automobile motor. Furthermore, the harnessing of power from, for example, water, wind, and human motion all involve the conversion of mechanical power into other forms of energy for more convenient storage and delivery. Typically, electrical generators are used in such applications.
In many electric vehicles, the same motor that develops the torque used in acceleration is also responsible for braking. During braking of these vehicles, electrical power is regenerated and batteries recharged. A major disadvantage of these electric vehicle motors, when considering both vehicle acceleration and power regeneration, is their low power to mass ratio for direct drive motors (&lt;0.2 kW/kg). Combustion engines, by comparison, produce 1 kW/kg, so that, although they are incapable of performing regeneration, they remain the motors of choice. Conducting polymer actuators offer the potential of power to mass ratios of 200 kW/kg.
Conducting polymers feature a conjugated carbon backbone. Some common conducting polymers are polyaniline, polypyrrole and polyacetylene. These materials are normally semiconductors. Without being bound to any particular theory, the conductivity of the conducting polymers may be changed by oxidation or reduction. It is theorized that the oxidation or reduction leads to an electric charge imbalance which, in turn, results in a flow of ions into the polymer material in order to restore balanced charge. Such ions or dopants may enter the polymer from a surrounding, ionically conductive, medium. The medium may be a gel, a solid electrolyte or a liquid electrolyte. If ions are already present in the polymer when it is oxidized or reduced, they may also exit the polymer. Such mass transfer of ions both into and out of the material leads to a contraction or expansion of the polymer. In some conducting polymers, the expansion is due to ion insertion between polymer chains; in others, interchain repulsion is the dominant effect. Additionally, conformational and bond length changes may also result in macroscopic expansion and contraction. Typical volume changes are on the order of 10%, and linear dimensional changes are hence on the order of 3%. Stresses have been observed in conducting polymer materials which are on the order of 20 Mpa, while bandwidths of 10 kHz are anticipated in conducting polymer actuators, although those observed to date are limited to 60 Hz.
Takashima et al. in Mechanochemoelectrical effect of polyaniline film, Synthetic Metals, 85, 1395 (1997) were the first to publish results demonstrating the use of conducting polymers in converting mechanical energy into electrical energy. They describe the generation of a current in response to deformation of a strip of polyaniline and attribute the current generated to a mechanically induced electrochemical reaction. The mechanical to electrical energy conversion efficiency is reported to be much less than 0.01%.